Tsukihito Amanuma (天沼 月人, Amanuma Tsukihito), also known as Gamemaster (遊熟者 (ゲームマスター), Yūjukumono (Gēmu Masutā), literally translated as Thoroughly-Wild Person), is a human who develops superhuman powers as a result of the demon plane hole opened by Sensui. Amanuma is able to bring video games to life, creating his territory within them. He is also a former member of the Sensui Seven.

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Amanuma is a young, fair skinned boy who has brown hair in a full fringe and at the back of his head it has a black bowl cut hairstyle.

He wears a sky blue long sleeved mini hoodie as the hood is blue, blue shorts, white socks and blue shoes. He also wears an arms guard on his right forearm, something frequently used to relieve stress on the forearm of his dominant hand.

In his initial meeting with Shinobu Sensui, Amanuma wears a red long sleeved hoodie as the color of his hood is white and light red with blue suspenders, a matching pants and shoes.

Fitting his age he is immature and carefree, thinking only of video games and being a little foul-mouthed. During his time alongside Sensui's Seven, Amanuma became arrogant and cruel, claiming to dislike the games he was prone to winning and mocking Mitarai for his change of heart, after the latter had joined Yusuke's friends in stopping Sensui. However, when Kurama reveals the nature of his power, and that he will die if he happens to lose the game, Amanuma quickly turns into a scared child.

Similar to Kiyoshi Mitarai, his former comrade among the Sensui Seven, Amanuma also describes himself as an outcast. While his school mates preferred to play sports, Amanuma spent his free time at arcades and TV consoles. However, after the events involving the near destruction of all humanity, Amanuma becomes nicer to children of his own age, befriending his school mates after realizing that they were not all that different. He becomes popular among them after they notice his talent with video games.

Amanuma didn't have any friends and wasn't close to his parents, so he played video games. Sensui once found him at an arcade and challenged him to a game. Tsukihito became interested after Sensui held him out to a draw, since he was the only person to have ever done so. Sensui offered him a chance to get back at everyone, not telling him that he and others, after using them, would die.

His job was to bring the game GoblinCity to life in order to slow the advancement of Yusuke's team towards Sensui. In order to win the game, they have to win four times, having seven chances. Seaman won the tennis game, Genkai the Battle Heli and Yusuke the King of the Hill. After the three wins, they must fight Amanuma/ Gamemaster for the fourth win. Kaito, confident because he has memorized the questions and the answers, volunteered for the Battle Quiz. Amanuma let him have the first five questions, secretly figuring out the pattern. After the five items, Amanuma pressed the button in the middle without having him or Kaito read the questions. Kaito was easily beaten after this. Amanuma later explained that if they lose the game, they can choose whether to die or to play again. Realizing the real purpose of this game, Kurama had to do the inevitable; He was forced to choose between one child and the human world. Kurama told Amanuma the truth- that Sensui was only using him. Shocked and not knowing what to believe, he lost his concentration which gave Yusuke's team the fourth win with Kurama beating the mini-game Three Sevens, and Amanuma as well.

According to the rules of the game, which applies to the real-life game that Amanuma made, the Gamemaster would die if beaten. After Kurama won, Amanuma died on the spot. His death shook Kurama and awakened a silent fury within him, which was shown when he took his anger out on Elder Toguro, seeing that the boy was being used by Sensui. Yusuke and the others advanced to the next room after that. Amanuma's body was left with the pick-up outside. It was later revealed that Koenma used his pacifier, or the Mafuuken to resurrect Amanuma. The death and the resurrection of Amanuma was one of Sensui's plans too. This was to lessen the power of the Mafuuken, his main threat. In the end, he was seen playing video games again. However, this time Amanuma began his new life with a different perspective. He started to make friends, and now goes to a private high school.

Territory: The ability to extend his Territory and bring video games to life. Whatever happens to the characters in the game happens to the one playing the character in real life. The effects of his powers somewhat differ in the anime and manga. In the manga, the players just play the arcade game with the machines, only with them taking the roles of characters in the video game (i.e. Player, Final Boss). In the anime, however, at certain times they literally enter the game with Genkai entering a helicopter in the flight sim game and Yusuke literally fighting the enemies in the Fighting game. Other times, (i.e. Kaito and Kurama) it plays out just like the manga, with two players in front of an arcade machine. Seaman 's game of tennis is different as he literally gets transported to the court and uses an arcade machine to control a duplicate tennis player of himself.

Driving: Amanuma is shown as an excellent driver when he drove a blue pick-up truck. It is very unusual to drive the car at a very young age of 12 despite it requires to drive the car at 18 years old and above. Amanuma demonstrates his driving skills when they are pursued by Yusuke who rides a bicycle until Sniper halts Yusuke's pursuit while chasing him in a motorcycle and destroys his bike by flicking with a single dice to destroy his bicycle and to make a diversion tactic to put Yusuke on his territory.

Since Sensui's Seven each represent one of the seven deadly sins, Gamemaster represents Sloth, in the way he prefers to laze about playing video games and how his power delays the heroes from getting to Sensui.

Tonda Birthday Present (1987, published in Weekly Shōnen Jump), one of Yoshihiro Togashi's earliest manga concepts, revolved around a young boy who accidentally brings a popular video game to life. It is possible that Amanuma's character and his power might have been loosely based off of this early work.

In Togashi's latest work, Hunter X Hunter, there exists a hunter-only video game called 'Greed Island' that supposedly transports the player into the game, but is later revealed to exist in the Real World, manifested through the abilities of the game's creators, who are referred to as 'Game Masters'. It is possible that this may be a reference to Amanuma and his Territory.

This power is similar to that possessed by the character Yukio Hans Voralberna from the popular anime and manga Bleach.

The popular anime and manga series Accel World and Sword Art Online have devices that can teleport the players into the games itself. This could be a reference to Amanuma and his Territory ability.

Like many other humans that have gained power from the demon gate, his psychic power puts Amanuma's threat level on the same level as A-class (though his spirit power is only D-class).

When they successfully captured Kuwabara and being pursuit by Yusuke, Amanuma is seen driving a car to escape and it is unusual to drove cars in his age.